German Patent No. 196 200 39 describes a method and a device for controlling an internal combustion engine with an actuator for influencing the quantity of recirculated exhaust gas. An actual value is compared with a setpoint. An actuator which influences the proportion of recirculated exhaust is driven on the basis of this comparison. With the device according to the related art, the air flow rate, in particular the amount of fresh air drawn in, is used as a controlled variable. It is compared with a setpoint which depends on the operating point of the internal combustion engine. Preferably a PI actuator corrects the control deviation.
In regulating low exhaust gas recirculation rates, the proportion of fresh air is large and the proportion of recirculated exhaust is low. In regulating the amount of fresh air, even a small sensor tolerance can lead to large relative fluctuations in the amount of recirculated exhaust and thus the exhaust gas recirculation rate.
For example, if an exhaust gas mass flow rate of 50 kg/h is to be mixed with a fresh air mass flow rate of 950 kg/h, and if the sensor measuring the amount of fresh air has a tolerance of .+-.5%, then an actual fresh air flow rate of approximately 900 kg/h or 1000 kg/h may be measured. Since the total fresh air mass flow and exhaust gas mass flow processed through the engine remains approximately constant, this leads to a recirculated exhaust gas flow rate of 0 kg/h or 100 kg/h.
Consequently, emissions are subject to great fluctuations. The emission characteristics at a low exhaust gas recirculation rate depend greatly on the accuracy in regulating the exhaust gas recirculation rate. At low exhaust gas recirculation rates, tolerances affecting the fresh air flow rate have very little effect on the exhaust gas recirculation rate. However, changes in exhaust gas mass have a great influence on the exhaust gas recirculation rate.
An object of the present invention is to reduce emissions with a method and a device for controlling an internal combustion engine. In particular, the accuracy in regulating the exhaust gas recirculation rate and the proportion of exhaust gas recirculated is to be improved.